DXOMark will provide you with accurately measured base ISO values for most DSLRs.

The base ISO of the D800 is actually 74, although Nikon describe it as 100. This is the ISO at which noise is lowest across the entire tonal range, and the ISO at which dynamic range and color sensitivity are highest.

There seems to be an anomaly with the 50D results. DXOMark describe the base ISO as being 158 at the manufacturer's specified ISO of 200, implying that ISO 100 is an expanded ISO which normally tends to clip highlights.

However, this does not quite accord with my own tests. ISO 100 with the 50D requires half the shutter speed of ISO 200, as one would expect, yet seems to produce identical results in respect of dynamic range and noise, with the exception of photonic shot noise, which appears to be very slightly and barely noticeably less in the ISO 100 shots, and then only barely noticeable at very high magnification.

For most practical purposes, I would recommend using ISO 200 (actually 158) as base ISO on the 50D, unless one has a reason to use a slower shutter speed.

DXOMark will provide you with accurately measured base ISO values for most DSLRs.

The base ISO of the D800 is actually 74, although Nikon describe it as 100. This is the ISO at which noise is lowest across the entire tonal range, and the ISO at which dynamic range and color sensitivity are highest.

If the base ISO is 74 or 100, and that is the ISO that is going to give optimal results, why does the camera select 250?

I would have thought that the engineers at Nikon would have programmed the camera to select the best possible ISO, if there is enough light to give a good shutterspeed/aperture combination. What reason would they have for not having the camera choose 100 (or 74)?

If the base ISO is 74 or 100, and that is the ISO that is going to give optimal results, why does the camera select 250?

I would have thought that the engineers at Nikon would have programmed the camera to select the best possible ISO, if there is enough light to give a good shutterspeed/aperture combination. What reason would they have for not having the camera choose 100 (or 74)?

Not arguing with you or DxOMark - just puzzled.

Ken

Probably because the camera is set, possibly by default, to use a minimum shutter speed of 1/250th when using auto ISO. I don't use P mode, but the general concept of using auto ISO is to ensure that the shutter speed is not too slow, rather than to ensure that one has the lowest, cleanest ISO.

For example, if I'm walking around taking shots of any subject that interests me, trying to capture fleeting moments when I may not have time to make manual changes to shutter speed or aperture, I might manually set the camera at a specific shutter speed and aperture beforehand, that I think would take care of most situations, such as F8 and 1/250th, and let the camera choose the ISO for an appropriate exposure. It's usually better to have a sharp image where subject movement has been frozen, than a blurry but clean image with maximum dynamic range.

If the base ISO is 74 or 100, and that is the ISO that is going to give optimal results, why does the camera select 250?

First off...the whole concept of Auto ISO completely ignores the concept of "native" or "optimal" ISO. The moment you select Auto ISO and Program mode, you've pretty much blown testing to "native" ISO out of the water...

If you want to test for native or optimal ISO you'll have to set the camera to manual and do a lot of exposure tests to determine the baseline ISO. Or check out DxO...everything you've done is a waste of effort...

First off...the whole concept of Auto ISO completely ignores the concept of "native" or "optimal" ISO. The moment you select Auto ISO and Program mode, you've pretty much blown testing to "native" ISO out of the water...

If you want to test for native or optimal ISO you'll have to set the camera to manual and do a lot of exposure tests to determine the baseline ISO. Or check out DxO...everything you've done is a waste of effort...

Okay - let's accept that I have wasted effort (actually, I didn't put in that much effort - took two photos) and that my test has no relevance to determining native ISO.

Leaving aside the question about native ISO, that still leaves the question as to why Nikon programmed their camera to use an ISO that is not optimal. What possible motive could they have had to choose to use ISO 250, when (according to DxO) the optimal ISO would be 100?

Most photographers I know, assuming there is enough light for a 'good' shutter speed and aperture, would select ISO100 rather than ISO250. The camera 'thinks' otherwise. Why does it do this?

Leaving aside the question about native ISO, that still leaves the question as to why Nikon programmed their camera to use an ISO that is not optimal. What possible motive could they have had to choose to use ISO 250, when (according to DxO) the optimal ISO would be 100?

You need to ignore the "selection" of ISO 250 with Auto ISO/Program mode...that's the part of your test that is totally, completely bogus...as far as I know, the baseline ISO of the D800/E is 100 with the "optimal' ISO prolly just a shade under ISO 100. Previous Nikon cameras' baseline ISO has been 200, not 100. Canon has generally stated a baseline ISO of 100.

Given a baseline ISO of 100, a given sample might be 100 or plus or minus 1/3 stop...which you can determine by testing but not by setting an Auto ISO nor program as the exposure...try using a manual exposure and determining the correct ISO by using a proper meter to measure the correct exposure for the scene...do you have a real exposure meter?

You need to ignore the "selection" of ISO 250 with Auto ISO/Program mode...that's the part of your test that is totally, completely bogus...as far as I know, the baseline ISO of the D800/E is 100 with the "optimal' ISO prolly just a shade under ISO 100. Previous Nikon cameras' baseline ISO has been 200, not 100. Canon has generally stated a baseline ISO of 100.

Given a baseline ISO of 100, a given sample might be 100 or plus or minus 1/3 stop...which you can determine by testing but not by setting an Auto ISO nor program as the exposure...try using a manual exposure and determining the correct ISO by using a proper meter to measure the correct exposure for the scene...do you have a real exposure meter?

I do have a 'real' exposure meter.

Being able to 'reverse engineer' the ISO from manual settings is not the question.

Let's assume there is enough light for 1/500sec, f/8 at ISO 100. Those are good numbers and I would be happy to use them. But, Nikon choose to rather either use settings like 1/1250sec f/8 or 1/500sec f/13 and increase the ISO to 250.

Everyone posting here has agreed that ISO100 is the optimal ISO.

Forget about native or base ISO. I know that was the initial question in the thread, but I have already acknowledged that the testing method may have no relevance. There is a new question that has evolved out of the discussion.

All I am now asking is ....Why do Nikon choose to bump up the ISO TO 250 rather than use the optimal 100? Somebody at Nikon programmed the camera to do this. Why did they do that? Is it just a bit of sub-standard programming by the software guys at Nikon or is there a logical reason why they did it?

All I am now asking is ....Why do Nikon choose to bump up the ISO TO 250 rather than use the optimal 100? Somebody at Nikon programmed the camera to do this. Why did they do that? Is it just a bit of sub-standard programming by the software guys at Nikon or is there a logical reason why they did it?

Perhaps they look at the way most people take photos (with a handheld and not a tripod mounted camera) and noticed that most people want to freeze action a bit more (if using Aperture priority) or have a bit more of the subject in focus (if shutter speed priority) or both (program mode) , and favored those pictorial qualities over the very very slightly lower signal to noise ratios you'd get at an ISO 100 setting.

...Leaving aside the question about native ISO, that still leaves the question as to why Nikon programmed their camera to use an ISO that is not optimal. What possible motive could they have had to choose to use ISO 250, when (according to DxO) the optimal ISO would be 100?

Most photographers I know, assuming there is enough light for a 'good' shutter speed and aperture, would select ISO100 rather than ISO250. The camera 'thinks' otherwise. Why does it do this?

Because the camera usually is less smart than the photographer and cannot guess what kind of parameters the photographer chooses for deciding at 'optimal' ISO and 'good shutter speed'.

Very basically ISO100 is the optimal ISO setting for just about every camera these days, in general it gives the best quality image.

The manufacturers are presumably making the assumption that anyone who is using AutoISO doesn't really no what they are doing and therefore sets the optimal settings that will guarantee the most success for getting a pretty decent photo.

Most cameras with auto settings do this. Its balancing the best of the capabilities of your average snapper.

If it didn't do this you'd have LOADS of lowly skilled photographers with no idea what they are talking about, but simply seeing the results that an image is blurred because they couldn't hold it steady at 1/125, and since they are only uploading them to facebook anyway its fine.